Géochimie isotopique du lithium dans les basaltes-Géochimie des ...

tel-00344949, version 1 - 7 Dec 2008
3. Article soumis à GCA en révision
�Parkinson et al., 2007�. In addition to these micrometer scale effects produced during
cooling of magmatic rocks, large scale Li isotopic variations have been recently recognized
to result likely from Li isotopic fractionation by diffusion during mantle melting and melt
infiltration �Lundstrom et al., 2005� or during hydrothermal fluid infiltration �Teng et al.,
2006�. Thus, though the variations in Li concentrations and in δ 7 Li values produced by
diffusion within the minerals of a cooling rock can be tentatively used to reconstruct the
cooling history of this rock �Coogan et al., 2005; Beck et al., 2006�, they are also a source of
strong complexity in Li isotope geochemistry in comparison with other light stable isotopes
such
as B, C, O.
94
Several questions are raised by these recent studies, concerning namely �i� the spatial
scale and the magnitude of Li isotopic variations due to diffusion in the case of common
magmatic rocks, �ii� the petrologic significance of the Li isotopic variations observed in
lavas, in their phenocrysts and their melt inclusions �Beck et al., 2004, 2006; Kobayashi et
al., 2004; Barrat et al., 2005; Jeffcoate et al., 2006; Halama et al., 2007; Parkinson et al.,
2007� and �iii� the implications of the Li isotopic variations in terms of cooling histories of
magmatic rocks. In order to answer to these questions we have selected a N‐MORB pillow
lava collected in the FAMOUS �French American Mid‐Ocean Undersea Study� area of the
Mid‐Atlantic Ridge. The sample consists of a "classic" fresh picrite that contains large
olivine crystals �up to 5 mm in diameter�. This picrite has been extensively studied for its
mineralogy, petrology, geochemistry and for the melt inclusions present in the olivine
phenocrysts �Bougault and Hekinian, 1974; Bryan, 1979; Le Roex et al., 1981; Kamenetsky,
1996; Laubier et al., 2007�. We have conducted a systematic search for Li isotopic
variations in the olivines by ion microprobe, trying to reconstruct the spatial distribution of
the isotopic variations in order to be able to better understand their origin and in
particular
to test the effect of Li diffusion between phenocrysts and glass.
2. SAMPLES AND ANALYTICAL PROCEDURES
A fragment of pillow sample ARP73‐10‐03, a typical picrite collected in the FAMOUS
area, was selected. Seven polished sections were cut at different depths through the pillow
lava from the glassy surface to the core and two of them were selected for the isotopic